The present invention relates to a data-transmitting apparatus for transmitting digital data representing various physical or chemical quantities measured by measuring devices, and more particularly, to a data-transmitting apparatus which converts such data into a serial-data message formed of signals, frequency-modulates these signals, and transmits the frequency-modulated signals.
In a factory, products are subjected to various inspections. An inspection line is provided in the factory to inspect the products. Measuring devices of various types are arranged along the inspection line. As the products are moved in the inspection line, their physical or chemical quantities are measured by means of the measuring devices. Pieces of data representing the physical or chemical quantities measured by the devices are collected in a host apparatus. The data collected in the host apparatus is used to calculate the quality and yield of the products.
There are various kinds of inspection performed on the products. One of them is the size-measuring of products. When the products have a complicated shape, ten or more dimensional values must be measured of each product. One person may use vernier calipers to measure all these dimensional values. Usually, however, several persons use vernier calipers or similar measuring devices, each person measuring a few dimensional values. This method is preferred, since the work efficiency is higher, and the error rate is lower, than when each person measures all dimensional values of the product. Hence, many measuring devices of the same type are required in most cases.
Data representing the values measured by the measuring devices is supplied through cables to, and is collected in, the host apparatus of a data collecting system. The data must be digital, or it cannot be processed in the host apparatus. Recently, a measuring device containing an analog-to-digital (A/D) converter has been put to practical use. The device measures a value of an article, and the A/D converter converts the data representing this value to digital data. The digital data is transmitted via cables from the device to the host apparatus. Since the data signals are digital, they are not attenuated, or degraded by noise. Hence, errors in the accuracy of the data can be minimized.
This digital data representing the value measured by the measuring device is frequency-modulated before it is transmitted to the host apparatus. More specifically, each digital data signal at "0" level is converted to a pulse signal of frequency F1, whereas each digital data signal at "1" level is converted to a pulse signal of frequency F2 higher than frequency F1. The data-transmitting apparatus used in the data collecting system outputs n pulses during period T for transmitting a "0" level, and m pulses during period T for transmitting a "1" level, where n&lt;m.
In order to perform the frequency modulation, the measuring device further contains two oscillators, one for generating pulses at frequency F1, and the other for generating pulses at frequency F2. The output pulses of these oscillators are transmitted under the control of an analog switch which is changed over in accordance with the levels of the digital data signals.
The measuring device, which a person holds in his or her hand and use to measure a value of an article, must be as small and light as possible. Since the device contains a battery for a power source, the power consumption of the device should be reduced to a minimum. Therefore, it is desired that the device be made of as few components as possible.
Each oscillator has a resonant circuit comprised of a capacitor and a resistor, or a capacitor and a coil. Hence, the measuring device, which is provided with two oscillators, is inevitably large and heavy, and is likely to consume much power.
The measuring device has another drawback. The resonant circuit (i.e., either oscillator), which is comprised of a capacitor and a resistor, or a capacitor and a coil, does not have a sufficient frequency stability. Thus, there is a high probability that the data is distorted and then transmitted. To reduce this probability, the oscillation frequency of either oscillator can be controlled by a phase-locked loop. The use of the phase-locked loop renders the measuring device even larger and heavier.